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Coming To Terms With Resolution!

Oct 23, 2005 at 12:00 am by deemarie

Discussion of the Term Resolution The term Resolution has various meanings depending upon the context in which it is used. This article will use scanners as input devices to illustrate different types of resolution. First, I will discuss raster images and resolution. A scanner takes an "electronic picture" of an image. The image can be reflective (paper) or transparent (film). Since this is a type of picture, it captures the image in bitmap or raster form. This form means that the output image is made up of pixels the same pixels viewed in Photoshop or Paint Shop Pro. Each pixel has a color depth. Usually one decides on how the image will be used, and then, on what output resolution is necessary. There used to be different output numbers such as 300, 200, etc. in manuals. Now, they have been replaced with terms such as good, better, best. These terms are ambiguous. Normally, if one is scanning for print and will use the whole image, 300 dots per inch (dpi) is a good choice. If one is scanning for the web, 72 dpi, also called screen resolution, is a good choice. Scanners are constructed employing two types of scanning resolution, hardware and software. Hardware resolution is the optical resolution. This means that the resolution is set through the optics of the scanner. One could call this a "pure" resolution. The other type of resolution that comes with a scanner or can be bought separately as a scanning package is software resolution. This starts where optical resolution leaves off. This type of resolution can be called interpolation, resampling, or upsampling. I will discuss this later. Various terms are also thrown around having to do with image resolution. These are ppi, spi, dpi, etc. ppi and spi - When scanners first became available to the home user, one could choose to scan through and specifying an input resolution. The term ppi means pixels per inch and spi means samples per inch. If one had a 4" x 5" picture, and one wanted an 8" x 10" image at 300 dots per inch, the input would have had to be 600 ppi or 600 spi, because the size of the output image would have been doubled. Now we rarely use that term in that context, and think of output resolution or dpi, which means dots per inch from printer terminology. Presently, the size of the input image is not as relevant. On most scanning devices, one can just put the output resolution desired and the computer software does the work. If you take a bitmap or raster image and magnify it, you will see a series of squares. These are pixels. They contain RGB information. The amount of the information is called bit-depth.
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When one starts a new image in Photoshop, or any other image editing or painting program, one can open a blank canvas and set the output image resolution and either the linear size in inches, millimeters, or the pixel linear dimensions. Depending on the pixel dimensions, the color depth, and a few other factors, the size of the image will be determined. Notice in the screen capture below the words: Image Size: 4.29M.
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One can then open this blank image in Photoshop and see all the information.
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The other alternatives: input an image through inputting devices such as scanners or digital cameras; or through exporting an image from a vector based program and changing that vector image to a raster image. However since many other factors are involved when formats are changed, I will not discuss the latter operation in this article.
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Let's assume that one has either created [or input an image] and its dimensions are 1/4" x 1/4" with a dpi of 72. The image is finished, but one wants to increase its linear dimensions to 1/2" x 1/2" and keep the dpi at 72. What does one do? One has to add pixels. Photoshop gives a choice on how one will either interpolate, upsample, or resample the image. All terms are used interchangeably. Message2444818.jpgWhile interpolating is accomplished very well now, the software still has to add pixels. The best way to understand this is to look at real images such as the bead on the upper left. That is why I am using such absurdly small sized images. The original size of the bead above was a little more than 1/4" or 20 pixels. And the resolution was 72 dpi. Look at the colors of the bead.
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In order to enlarge the image of the bead, not magnify it, but virtually enlarge it, I had to use interpolation. It was enlarged to 40 pixels wide, keeping the dimensions constant which turned out to be a little over 1/2" x 1/2". Had I enlarged it to exactly 1/2", the pixel linear dimensions would have been different.
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Again, notice the colors in the bead. And remember that the software had to figure out the exact colors to add. What was the point of this whole exercise? Just look at the two pictures of the beads and ask yourself how the colors for the additional pixels were chosen. Are they positively accurate? Basically, using good programs like Photoshop, for most work, one can resample upwards or sample up carefully as long as one keeps in mind that software is adding pixels. Where it can become difficult is with color separation printing; where one is working with a 72 dpi image; and it has to be made into a 300 dpi image; and a lot of pixels have to be added. A program such as Genuine Fractals 4 Pro uses a totally different method of increasing the number of pixels and is safer to use than trying to do it oneself if the increase is a lot. However, if an image is going to be printed using color separation, and requires a lot of interpolation, the best bet is to speak to whomever is doing the final output and let them advise you on what to do.
All supporting images are copyright, and cannot be
copied, printed, or reproduced in any manner without written permission from the artist.
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  • The Paula Sander's Report is a regular Renderosity Front Page featured column, where Paula investigates and comments on graphic software, techniques, and other relevant material through her reviews, tutorials, and general articles.
October 24, 2005
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